Bearings used in modern engines need to possess a combination of often conflicting properties. Bearings generally comprise several layers: a backing layer of a strong material such as steel or bronze having a thickness in the range from about 1 to about 10 mm; a layer of a bearing alloy usually based upon alloys of copper or aluminium and having a thickness in the range from about 0.1 to about 1 mm; and, optionally, a so-called overlay layer on top of the bearing alloy layer and having a thickness in the range from about 5 to about 50 μm. There may also be additional layers: one situated between the backing and the bearing alloy layer to assist in enhancing adhesion between these two layers and comprising, for example, a thin layer (generally about 5 to 50 μm, although much thicker layers up to about 300 μm have been known) of aluminium or an aluminium alloy, nickel or another material as are known in the art. A further layer, a so-called interlayer, may be situated between the bearing alloy layer and the overlay layer and be present for the purpose of enhancing adhesion between the two layers and/or acting as a barrier to prevent or minimise unwanted diffusion of alloying constituents between the two layers. Such interlayers are usually very thin, of the order of about 0.5 to 5 μm.
Where present, the overlay layer provides the actual running or sliding surface between the bearing itself and a co-operating shaft journal. The overlay is generally a relatively soft material being based upon alloys having tin, lead, cadmium or aluminium as their main constituent. The purpose of the overlay, which is generally softer than the bearing alloy layer, is to provide a conformable layer able to accommodate small misalignments between the bearing and shaft journal caused due to imperfections in the machining processes involved in the bearing and engine manufacturing processes, i.e. the overlay possesses the characteristic of conformability. The overlay layer must also be seizure resistant, fatigue resistant, corrosion resistant; wear resistant and provide for embeddability of dirt and debris carried in the lubricating oil. Good fatigue resistance and wear resistance are generally associated with high strength and hardness. Good seizure resistance requires the material forming the running surface to have good compatibility which overlay alloys, due to their composition, generally possess. Similar requirements are also associated with the bearing alloy layer where no overlay is present and the bearing alloy itself forms the actual running or sliding surface. However, it should be borne in mind that in some engines, due to the arduous service conditions, it is common for the overlay layer to be worn away on at least part of the sliding surface (generally in a loaded area) thereby exposing the underlying bearing alloy layer which then becomes the actual sliding or running surface.
It is known, for example from WO 99/47723 and WO 2006/035220, the full contents of which are hereby incorporated into the present application by reference, to provide bearing alloy layers and overlay coating layers based on aluminium and alloys thereof by way of a High Velocity Oxy-Fuel (HVOF) spraying process.
It is also known, when manufacturing split bearings generally comprising two semi-cylindrical half shells which are intended for assembly around a journal member (such as a crankshaft) or a slide member (such as a push rod or connector rod), to provide a bore relief or crush relief portion (hereinafter referred to generally as a crush relief) along the inner longitudinal edges of the half shells. The crush relief, which takes the form of a narrowing of the wall of the bearing half shell in a region adjacent to the parting face of the bearing, is generally formed by removing part of the bearing lining by a machining process, such as high speed boring. A typical example of a split journal bearing having crush relieves is disclosed in US 2005/0196084, the full content of which is hereby incorporated into the present application by reference. The crush relief is provided on the bearing surface of split journal bearings to accommodate any slight deformation or misalignment caused when the two bearing half shells are forced into engagement.
With reference to FIG. 1, It is known to manufacture a bearing half shell 1 by starting with a generally rectangular hard metal (e.g. steel or other hard alloy) blank with a softer layer of a bearing alloy such as a copper-based material formed thereon. The blank is then pressed or stamped so as to deform it into the required semi-cylindrical shape, with the softer layer of the bearing alloy 3 forming the interior lining surface of the half shell thus formed, and the harder metal 2 of the substrate forming the outer surface. The half shell 1 is then subjected to an initial rough machining, followed by more precise boring to a precise thickness, including the formation of crush relief portions 4. The crush relieves 4 are formed in the bearing lining 3, which means that the bearing lining 3 will be thinner in the region of the crush relieves 4 than elsewhere on the lining surface. An overlay 5 is then sprayed onto the interior surface of the half shell by way of an HVOF process, with the thickness of the overlay 5 being relatively constant over the whole interior surface. The half shell 1 is then subjected to final machining and boring, including the crush relief portions 4, to remove a top surface 6 of the overlay 5, leave a bearing half shell with the precise bore thickness and crush relief dimensions that are required, and a generally smooth bearing surface 6. It will be noted that, even after final machining and boring, the entire exposed bearing surface 6 will be comprised of the overlay 5 applied by HVOF spraying.
FIG. 2 shows, in schematic form and without identifying the layers, a section of the half shell 1 with an outer backing surface 7 having an outer diameter dimension Douter with respect to a longitudinal axis of the half shell and an inner bore surface 8 having an inner diameter dimension Dinner smaller than the outer diameter dimension Douter. The crush relief 4 is formed separately, and has a crush relief dimension d defined by the depth of the overlay removed at the joint face 9.